As methods of converting AC voltage to DC voltage, following two methods are generally known.
In the first method, a diode bridge circuit and a smoothing capacitor are used. The diode bridge circuit full-wave rectifies an alternating current from an AC power supply. The smoothing capacitor smoothes a direct current after full-wave rectification.
In the first method, in either case where the AC voltage is positive or negative, current always flows through a series circuit of two diodes. At this time, in each of the two diodes, power loss corresponding to the product of the current flowing through the diode and the forward voltage of the diode occurs.
In the second method, a power factor improving converter (PFC) intervenes between the diode bridge circuit and the smoothing capacitor of the first method. The power factor improving converter raises the DC voltage full-wave rectified by the diode bridge circuit.
Also in the second method, since the current flows through the series circuit of two diodes at the time of full-wave rectification, power loss occurs. In addition to that, since current alternately flows through a field effect transistor (FET) constituting the power factor improving converter and a diode, further loss occurs.
Besides, in order that the power factor improving converter converts the waveform of an input current into a sine wave, the output voltage must be set to be higher than the input voltage. However, a voltage required for a load is not necessarily a voltage higher than the input voltage. In that case, a step-down converter is connected to the latter stage of the power factor improving converter, and the voltage raised by the power factor improving converter is reduced to a desired voltage. Also when the voltage is reduced, a loss occurs. The whole power conversion apparatus includes three stages of the AC-DC conversion, the DC-DC (step-up) conversion and the DC-DC (step-down) conversion, and the power loss appears as a product of these. For example, if the efficiency of one stage is 0.95, the efficiency of the three stages becomes 0.95×0.95×0.95=0.86. That is, even if the conversion efficiency is as high as 95%, the efficiency is reduced to 86% in the three-stage connection. As described above, even if the conversion efficiency of each stage is high, the conversion efficiency extremely decreases in multiple stages.
Recently, energy saving of electronic equipments is demanded, and as part thereof, the improvement of the conversion efficiency of a power conversion apparatus to supply power to a load is required. However, the improvement of the conversion efficiency is limited in the related art circuit structure.